Professor Paul D. Beer

Research

My general field of research is host-guest supramolecular chemistry, which covers a broad area of interests.

With a view to increasing the understanding of molecular recognition processes in biological systems and producing new molecular sensors, switches and devices, my research is focused on the synthesis of novel macrocyclic and interlocked host molecules that contain redox- or photo-active reporter groups. These systems have been designed to complex and sense cationic, anionic or neutral inorganic or organic guest species via electrochemical and optical methods. Selective binding of a particular guest species is of paramount importance for commercial applications such as potential prototypes of new molecular sensory devices, molecular switches and extraction agents for cleansing the environment of toxic materials.

Specific research topics of current interest include:

• Rotaxane and Catenane Host Systems: The synthesis of novel interlocked molecular architectures using anionic species as templating motifs is a key area of our research. Using this methodology, we have constructed a range of novel pseudo-rotaxanes, rotaxanes and catenanes, wherein an anion directs the interpenetration process. The crystal structure below, for example, displays a [2]catenane in which the templating chloride anion remains encapsulated within the interlocked motif. After halide anion template removal, these interlocked host systems have the potential, by virtue of their unique topological cavities, to exhibit unprecedented anion recognition, sensing and molecular machine-like properties, whereby anion binding controls molecular movement of the rotaxane/catenane constituent parts.

• Sensors for Anionic Guest Species: The synthesis of novel anion sensors is achieved through the attachment of redox- and photo-active reporter groups, such as ferrocene, transition metal luminophores and emissive lanthanide complexes, to selective anion receptors. Careful design enables these sensor molecules to optically or electrochemically detect anionic guest species of particular biological and environmental importance, such as phosphates, nitrates, halides and carboxylates. The topology of the host cavity is designed to complement the target anion and results in the desired selectivity.

• Surface and Nanoparticle Based Anion Sensors: There has been an explosion of interest in the area of surface and nanoparticle chemistry in recent years, due to the singular optical and electrochemical properties of nanoparticles and their applications in catalysis, biomedical imaging and materials. In collaboration with Professor Jason Davis' group, our research in this area focuses on exploiting the remarkable surface enhancement of anion recognition to fabricate highly sensitive and selective anion detection devices. For example the surface assembled redox-active rotaxane displayed below selectively senses chloride ions electrochemically.

• Ion Pair Recognition: This exciting area of coordination chemistry is concerned with the syntheses of host molecules that contain binding sites for both anionic and cationic guest species. These host systems are designed to be selective for target metal salts, for example sodium chloride in the ion-pair rotaxane host below, and zwitterionic guests such as amino acids. The simultaneous binding of toxic/radioactive ion-pair species could make these systems novel extraction reagents for the purification of industrial effluent, soil water and radioactive waste streams.

• Halogen bonding: Of the non-covalent interactions employed in supramolecular chemistry, halogen bonding is largely underexploited, with the majority of cases coming from solid state crystal engineering material applications. We have reported the first examples of solution phase halogen bonding being exploited to recognize and sense anions in aqueous media, and to control and facilitate the anion templated assembly of rotaxane and catenane interlocked structures. For example the rotaxane displayed below can recognise halide anions in water with remarkable affinity, up to two orders of magnitude greater than in the analogous hydrogen bonding system. Research is currently underway to extend the scope of solution phase halogen bonding in supramolecular chemistry.

• Anion recognition-induced molecular motion: Mechanically interlocked molecules (MIMs) are firmly established entities in the field of nanoscale molecular machines because of their ability to undergo controlled and reversible molecular motion through changes in the relative positions of their constituent parts. By careful design the inherent dynamics of such systems may be governed by a variety of stimuli, however, co-conformational switching mediated by the recognition and sensing of anionic species is underexplored. This concept has been demonstrated with the construction of an exotic XB [3]rotaxane four-station molecular shuttle that is capable of the colorimetric sensing of oxoanions, in particular nitrate, courtesy of novel pincer-like motion of the two macrocycle components that occurs upon binding.